This invention relates to a method of applying polyimide to a substrate in general, and particularly to a method in which liquid polyimide is adhered to the substrate.
Polyimide, due to its electrical and mechanical properties, is a desirable material for use on substrates, both as a base, as well as a material for use in providing layers of a multi-layer circuit board, and as a cover for the metal conductors. However, it is difficult to obtain good adhesion between liquid polyimide and cured polyimide, metals such as gold, and substrate materials. It is desirable to be able to build up layers of polyimide; however, known prior art approaches where multiple layers of polyimide are applied, are subject to cracking and delaminating of the polyimide, both over metallic runners and pads, such as gold and between layers of polyimide. In addition to poor adhesion to gold metal and cured polyimide resin, these surfaces also cause the liquid polyimide to dewet, creating voids and pinholes in the coating. In order to apply polyimide coatings to substrates, and build up successive layers of polyimide, these problems must be eliminated. The present method of applying liquid polyimide utilizes nucleation sites to provide adhesion promotion and enhanced wetting between the liquid polyimide and the materials to which it is applied.